Electron gun for cathode-ray tubes



Oct. 30, 1951 c. s. SZEGHO I ,-2,573,287

ELECTRON GUN FOR CATHODE=RAY TUBES Filed June 23, 1950 CONSTANTIN S. SZEGHO INVENTOR.

H/S ATTORNEY Patented Oct. 30, 1951 ELECTRON GUN FOR CATHODE-RAY TUBES Constantin S. Szegho, Chicago, Ill., assignor to The Rauland Corporation, a corporation of Illinok Application June 23, 1950, Serial No. 169,995

Claims. (Cl. 313-41) This invention relates to cathode-ray tubes suitable for use as the image-reproducing device of a television receiver and relates, more particularly, to an improved cathode-ray tube in which means is provided to prevent the-formation of negative ions which have a deleterious effect on the reproducing screen therein.

In cathode-ray tubes utilized in present-day television receivers, an electron beam is developed by an electron gun mounted at one end of the tube and is directed thereby to a fluorescent screen disposed at the opposite end. In many instances the electron beam insuch tubes is defiected over the screen area by means of an electro-magnetic yoke which develops varying magnetic fields within the tube in response to periodic lineand field-scanning signals. when magnetic deflection is utilized, a dark spot usually appears on the screen after the tube has been in operation for some time. This spot is caused by the bombardment of negative ions which originate at the cathode of the electron gun and which have a mass that is materially greater than that of the electrons. These negative ions result from the bombardment of the cathode by positive ions which, in turn, are produced by the electrons of the cathode-ray beam in their travel from the cathode to the screen. Since the cathode is maintained at a highly negative potential relative to the other electrodes, these positive ions are attracted to the cathode and impinge thereon with a sufilciently high velocity to release negative ions which are attracted to and damage the fluorescent screen as mentioned above.

The accelerating fields of the cathode-ray tube accelerate the negative ions along with the electrons constituting the cathode-ray beam although the negative ions travel at a slower rate than the electrons due to their greater mass. It is well known that the force exerted on a charged particle by a magnetic field is a function of the velocity and hence of the mass of this particle. For this reason, the electrons of the cathode-ray beam are readily deflected over the entire screen area of the cathode-ray tube by the magnetic field set up therein by the yoke, but the negative ions are not affected by this field to any appreciable extent. Therefore, the negative ions impinge upon a very limited portion of the screen area, usually in the immediate vicinity of the undeflected beam position and after the tube has been in use for some time, this impingement causes contamination of the screen and gives rise to an "ion spot" at the point of impingement.

Several arrangements have been proposed for preventing the harmful negative ions from reaching the screen of the cathode-ray tube. These arrangements usually utilize the combined efiects of electro-static and magnetic fields to trap the ions. An electro-static field is used to deflect the mixed beam of electrons and ions away from the fluorescent screen towards a trappin Surface, while a magnetic field is used to return solely the electrons of the beam to a path directed to the fluorescent screen.

The present invention provides a cathode-ray tube that is constructed so that no appreciable amount-of negative ions is developed therein and there is therefore no need for additional components to provide electro-static and magnetic fields to trap the negative ions.

It is, accordingly, an object of this invention to provide an improved cathode-ray tube that is so constructed that substantially no undesirable negative ionsare developed within the tube.

Another object of this invention is to provide such an improved cathode-ray tube including an electron gun that operates with a high degree of efficiency to provide an ion-free electron beam. and yet is simple and economical to construct.

The features of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description when taken in conjunction with the accompanying drawing, in which the single figure shows one embodiment of the improved cathode-ray tube of the present invention.

Referring now to the drawing, there is illustrated a section It of the neck portion of a cathode-ray tube. The neck It! may have a usual base I I afiixed thereto which, in turn, has a plurality of pins l2 extending therethrough and electrically connected to various elements of an electron gun mounted within the tube. These elements may be supported by glass rods 13 extending longitudinally within the tube and rigidly held therein by any suitable means. More specifically. the electron gun includes a tubular cathode ll which is electrically connected to a first coaxial beam-forming electrode IS. The cathode It and electrode l5 may be supported in coaxial alignment with the longitudinal axis of the tube by means of lugs l6 embedded in rods l3. The upper surface of cathode I4 is, preferably, cup shaped and activated with electronemissive material. Cathode l4 and beam-forming electrode I5 are surrounded by a second co- 3 axial beam-fanning electrode I'I having lower transverse portion I 8 with a central aperture is therein. Electrode I! may be supported from rods l3 by means of radial lugs 20 extending from the electrode and embedded in the rods. A tubular accelerating anode electrode 25 may be mounted within the tube in coaxial alignment with electrode 11. The anode 25 is supported from rods It by means of radial lugs 28, and has its upper end enclosed by a transverse disc 21 having a central aperture 28.

In accordance with the invention. a central portion 30 of the upper surface of cathode H is rendered non-emissive. The activated surface of cathode I may be heated to an electron-emissive temperature by any suitable internal heater element (not shown) or, when so desired, the oathode may be heated by electron bombardment as disclosed, for example, in British Patent 460,445 January 27, 1937, to Szegho.

The principles governing the configuration of cathode l4 and the arrangement of this cathode and electrodes l5 and I! are described in a text book by J. R. Pierce entitled "Theory and Design 2 of Electron Beams," published in;1949 by, the Van Nostrand Company; and in Patent 2 268,197 to Pierce. The first beam-forming electrode I5 is established at cathode potential and the sectial superposed on its positive potential to modulate the intensity of the electron beam infaccordance with television practice, or when desired an additional modulating electrode may be included in the tube. It is desirable to use this type of construction of cathode I and electrodes l5 and I1 since it. enables the entire activated surface of the cathode to be utilized in the formation of the electron beam, and the non-emissive portion has no material effect on the emission efllciency of the cathode. Q The anode electrode 25 may have a suitable potential impressed thereon to provide an accelerating field for the electron beam to direct the beam along the tube axis and through aperture 28.

As previously pointed out, the electron beam develops positive ions as it travels the length of the cathode-ray tube from cathode l4 tothe fluorescent screen. These positive ions are attracted to cathode ll due to the fact that the cathode is at a negative potential with respect to electrodes l1 and 25. The ions develop a relaively high kinetic energy as they approach the cathode surface and are not deviated by the beam-forming electrodes. Therefore, such ions impinge on cathode portion 30 which. as previously described, is not emissive. Consequently, no significant amount of negative ions are released by the cathode and there is no necessity for any ion trapping arrangement within the tube.

As previously pointed out, the disclosed configuration of cathode I4 and electrodes I5 and I1 is preferred since the non-emissive central portion 30 has little eflect on the emission efflciency of the cathode due to the fact that the entire activated surface is used to form the electron beam. If desired, the non-emissive poramass? tion 80 may take the form ofan aperture in the cathode surface so that the positive ions enter the cathode and are dissipated by the internal surface thereof. It is preferable to place a blocking mask behind the aperture of high melt.- ing point material and of low secondary emission such as, for example, tungsten. The area of portion 30 is made-sufiiciently large relative to the full cathode surface that substantially all the positive ions returning to the cathode impinge thereon, yet this portion is made sufllciently small that cathode efllciency is not materially impaired. For example, should the surface of cathode I have an area of .05 square inch, portion 30 may have a diameter of substantially for satisfactory results.

The cathode-ray tube also includes an enlarged or bulbous portion 40 having a fluorescent screen 4| affixed to its extremity for scansion by the cathode-ray beam developed by the electrongun structure. The tube may have associated therewith magnetic beam-deflection elements 42 for scanning the cathode-ray beam over a twodimensional area of screen 4!. Any well-known type of magnetic focusing arrangement may also be mounted on the tube for focusing the cathode-ray beam into a well-defined spot'on screen within the cathode-ray tube and the possibility of contamination of screen ll thereby is preeluded. 1

While a particular embodiment of the invention has been and described modifications may be made and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention. I claim:

1. A cathode-ray tube of the image-reproducing type comprising: a fluorescent screen supported within said tube at one end of an electrondischarge path; magnetic deflection elements for scanning an area of said screen with a cathoderay beam developed at the opposite end of said tube and which beam is subject to produce positive ions therein in its travel to said screen;

a cathode electrode supported at said opposite end coaxially with respectto said discharge path and having an electron-emissive surface extend- 'mounted within said tube surrounding said path and disposed between said cathode and said screen for forming electrons emitted from said surface of said cathode into a beam and for accelerating said beam along said path.

2. A- cathode-ray tube of the image-reproducing type comprising: a fluorescent screen supported within said tube at one end of an electrondischarge path; magnetic deflection elements for scanning an area of said screen with a cathoderay beam developed at the opposite end of said tube and which beam'is subject to produce positive ions therein in its travel to said screen; a cathode electrode supported at said opposite end coaxially with respect to said discharge path and having an electron-emissive surface extending transversely of said path but with a centrallylocated non-emissive portion exposed to said path to receive said positive ions attracted to said surface along said path; a first beam-forming electrode positioned coaxially with respect to said path in the vicinity of said cathode surface; a second beam-forming electrode positioned coaxially with respect to said path between said screen and said first beam-forming electrode? and terminal connections extending to said electrodes for establishing a difference of potential therebetween to form electrons emitted from said surface of said cathode into a beam and to accelerate said beam along said path.

3. A cathode-ray tube 01' the image-reproducing type comprising: a fluorescent screen supported within said tube at one end of an electrondischarge path; magnetic deflection elements for scanning an area of said screen with a cathoderay beam developed at the opposite end of said tube and which beam is subject to produce positive ions therein in its travel to said screen; a cathode electrode supported at said opposite end coaxially with respect to said discharge path and having an eleetron-emissive surface extending transversely of said path but with a centrallylocated non-emissive portion exposed to said path to receive said positive ions attracted to said surface along said path; a first tubular beamforming electrode positioned coaxially with respect to said path in the vicinity of said cathode surface; a second tubular beam-forming electrode positioned coaxially with respect to said path and interposed between said screen and said first beam-forming electrode: and terminal connections extending to said electrodes for establishing a difference of potential therebetween to form electrons emitted from the surface of said cathode into a beam and to accelerate said beam along said path.

4. A cathode-ray tube of the image-reproducing type comprising: a fluorescent screen supported within said tube at one end of an electrondischarge path; magnetic deflection elements for scanning an area of said screen with a cathoderay beam developed at the opposite end of said tube and which beam is subject to produce positive ions therein in its travel to said screen; a cathode electrode supported at said opposite end coaxially with respect to said discharge path and having a cup-shaped electron-emissive surface extending transversely of said path but with a centrally-located non-emisive portion exposed to said path to receive said positive ions attracted to said surface along said path; a first beami'orming electrode positioned coaxialiy with respect to said path in the vicinity of said cathode surface; a second beam-forming electrode positioned coaxially with respect to said path between said screen and said first beam-forming electrode; and terminal connections extending to said electrodes for establishing a difference or potential therebetween to form electrons emitted from said surface of said cathode into a beam and to accelerate said beam along said path.

5. A cathode-ray tube of the image-reproducing type comprising: a fluorescent screen supported within said tube at one end of an electrondischarge path; magnetic deflection elements for scanning an area of said screen with a cathoderay beam developed at the opposite end of said tube and which beam is subject to produce positive ions therein in its travel to said screen; a cathode electrode supported at said opposite end coaxially with respect to said discharge path and having a cup-shaped eleetron-emissive surface extending transversely of said path but with a centrally-located non-emissive portion exposed to said path to receive said positive ions attracted to said surface along said path; a first tubular beam-forming electrode positioned coaxially with respect to said path in the vicinity of said cathode surface; a second tubular beam-forming electrode positioned coaxially with respect to said path and interposed between said screen and said first beam-forming electrode; a tubular anode positioned coaxially with respect to said path having a transverse end wall with a central aperture thereimand terminal connections extending to said electrodes for establishing a difierence of potential therebetween to form electrons emitted from the surface of said cathode into a beam and to direct said beam through said aperture.

CONSTANTIN S. SZEGHO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,378 Bowie Sept. 21, 1943 2,100,701 Schlesinger Nov. 30, 1937 2,276,806 Tonks Mar. 17, 1942 2,308,800 Anderson Jan. 19, 1943 2,321,886 Anderson June 15, 1943 2,429,824 Koch Oct. 28, 1947 2,460,738 Francis Feb. 1, 1949 2,460,739 Francis Feb. 1, 1949 2,466,064 Wathen et al. Apr. 5, 1949 FOREIGN PATENTS Number Country Date 358,978 Great Britain Sept. 17. 1931 

